1. Technical Field
Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a fixing method, and more particularly, to a fixing device for fixing a toner image on a recording medium, an image forming apparatus incorporating the fixing device, and a fixing method for fixing a toner image on a recording medium.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a fixing rotary body heated by a heater and an opposed body contacting the fixing rotary body to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. As the fixing rotary body and the opposed body rotate and convey the recording medium bearing the toner image through the fixing nip, the fixing rotary body heated to a predetermined fixing temperature and the opposed body together heat and melt toner of the toner image, thus fixing the toner image on the recording medium.
Since the recording medium passing through the fixing nip draws heat from the fixing rotary body, a temperature sensor detects the temperature of the fixing rotary body to maintain the fixing rotary body at a desired temperature. Conversely, at each lateral end of the fixing rotary body in an axial direction thereof, the recording medium is not conveyed over the fixing rotary body and therefore does not draw heat from the fixing rotary body. Accordingly, after a plurality of recording media is conveyed through the fixing nip continuously, a non-conveyance span situated at each lateral end of the fixing rotary body may overheat.
To address this circumstance, the fixing device may incorporate a heat shield to shield the non-conveyance span of the fixing rotary body from the heater, thus preventing overheating of the fixing rotary body as disclosed by JP-2008-058833-A and JP-2008-139779-A, for example. The heat shield is movable to shield the fixing rotary body from the heater in a variable span on the fixing rotary body according to the size of the recording medium.
However, if the heater and other interior components are situated inside the fixing rotary body, those components may create a direct heating span on the fixing rotary body where the heater is disposed opposite the fixing rotary body directly and an indirect heating span on the fixing rotary body where the heater is disposed opposite the fixing rotary body indirectly through those interior components. As the heater is turned on, the direct heating span on the fixing rotary body is heated to an increased temperature. Conversely, the indirect heating span on the fixing rotary body is heated to a decreased temperature. Thus, the heater may heat the fixing rotary body unevenly.
Even after the fixing rotary body rotates idly for a while, unevenness of temperature of the fixing rotary body may not be eliminated. For example, when the fixing device is warmed up from a decreased temperature, the opposed body having an increased thermal capacity may draw heat from the fixing rotary body heated by the heater. Accordingly, even after the fixing rotary body rotates idly for an extended period of time, unevenness of temperature of the fixing rotary body may not be eliminated.
Uneven temperature of the fixing rotary body may thermally expand the fixing rotary body locally, causing warping and deformation on the surface of the fixing rotary body which may obstruct formation of the fixing nip between the fixing rotary body and the opposed body. Hence, the fixing rotary body and the opposed body may not apply heat and pressure to the recording medium conveyed through the fixing nip properly.
If the movable heat shield is retained at a halt position where it is halted when the previous print job is finished, the position of the heat shield when the next print job starts may vary depending on the halt position of the heat shield when the previous print job is finished. Accordingly, it may be difficult to adjust the temperature of the fixing rotary body to an even temperature during each print job, causing warping and deformation of the fixing rotary body.